1. Field of the Invention
The present invention relates to a statistical multiplexing apparatus in a time division multiplexing bus, and more particularly to apparatuses for statistically multiplexing in a time division multiplexing bus by using a priority to be appropriate to an asynchronous transfer mode.
2. Description of Related Art
In a conventional time division multiplexing (hereinafter referred to as a "TDM") apparatus as shown in FIG. 1, a plurality of subscriber units 1.sub.1, 1.sub.2, . . . 1.sub.n and a trunk unit 2 are connected via a transmission bus Tx BUS and a reception bus Rx BUS.
Each subscriber unit is comprised of a time slot generator 110 for generating a time slot in response to a clock pulse CLOCK and outputting an enable signal EN to a reception buffer 112 and a bus interface unit 113; a transmission buffer 111 for buffering a subscriber data and outputting it to the transmission bus Tx BUS via the bus interface unit 113; the bus interface unit 113 for interfacing the subscriber data to the transmission bus Tx BUS and the data on the reception bus Rx BUS to the reception buffer 112 in response to the enable signal EN output from the time slot generator 111; and the reception buffer 112 for outputting the data from the bus interface unit 113 according to the status of the enable signal EN output from the time slot generator 110.
The conventional TDM apparatus, as described above, time-divides into n time slots TS.sub.1 to TS.sub.n, as shown in the TDM bus configuration of FIG. 3, to multiplex n subscribers.
The multiplexing operation will be discussed with reference to FIG. 2. The time slot generator 110 generates the enable signal EN appropriate to the time slot of a corresponding subscriber. The subscriber data is transmitted to the transmission bus Tx BUS via the transmission buffer 111 and the tri-state buffer BF.sub.1 within the bus interface unit 113. In this case, the output of the tri-state buffer BF, is controlled by the enable signal EN output from the time slot generator 110.
In the meanwhile, the data input to the reception bus Rx BUS is transferred to the subscriber via the buffer BF.sub.2 within the bus interface unit 113 and the reception buffer 112.
However, in a conventional apparatus, the number of the subscribers that one trunk can accommodate is determined by n time slots of the TDM bus. For example, T.sub.1 has 24 time slots and E.sub.1 has 30 time slots. The more the time slots, the higher the system clock frequency, resulting in the limited number of the time slots.
Another problem is that even when a corresponding subscriber does not transmit any data; the time slot cannot be used by another subscriber, thereby deteriorating the efficiency of the bandwidth.
In order to solve these problems, a conventional technique disclosed in U.S. Pat. No. 5,526,345 proposes a method for statistically multiplexing by managing a leaky bucket manager, a cell buffer and a message cell memory when a plurality of message cells are input and then are output to one output terminal in an asynchronous transfer mode (hereinafter referred to as an "ATM"). However, this technique also has a problem of complicated configuration and lower data processing speed.